The present invention relates to a driving circuit for a piezoelectric transformer, widely used as a step-up transformer for driving the backlight of a liquid crystal display monitor, and the like.
In recent years, liquid crystal display monitors have been incorporated in portable compact video apparatuses, such as a VCR integrated with a camera and a digital camera. As a step-up transformer for driving a cold cathode tube widely used for the backlight etc. of a liquid crystal display monitor of compact video apparatus, a piezoelectric transformer which can be made thinner, has higher efficiency and can be free from the generation of magnetic line forces has begun to be used in place of an electromagnetic transformer having been used conventionally. The piezoelectric transformer is a voltage conversion device, wherein an input AC voltage is applied to the primary electrode of a piezoelectric device, mechanical vibration is generated by using a piezoelectric effect, and a voltage amplified depending on a voltage step-up ratio determined by the shape of the piezoelectric transformer is taken out from the secondary electrode thereof. The piezoelectric transformer does not cause any leak magnetic fluxes because it does not use a method wherein voltage is changed via magnetic energy by using windings. For this reason, it is advantageous in not causing noise outside its inverter. In addition, since the piezoelectric transformer selects and outputs only the frequency determined by its outer dimensions, its output waveform is close to a sine wave, thereby being advantageous in reducing the occurrence of high-frequency noise. Furthermore, since the piezoelectric transformer is a mineral formed by sintering a ceramic material, it has the advantage of causing no risk of smoking and ignition.
FIG. 61 is a graph showing the general characteristic of a piezoelectric transformer, the abscissa represents the frequency [Hz] of an input voltage, and the ordinate represents an output value [dB].
As shown in FIG. 61, the piezoelectric transformer has a resonance characteristic, and the output value obtained from the secondary electrode differs depending on the frequency of the AC voltage input to the primary electrode. Therefore, in the piezoelectric transformer, in order to control the brightness of the backlight at a constant level, the voltage output from the secondary electrode can be adjusted to a desired level by controlling the frequency of the AC voltage input to the piezoelectric transformer. As described above, the voltage having the desired level is output from the secondary electrode of the piezoelectric transformer, whereby a stable voltage is applied to a cold cathode tube. A typical technology for the driving circuit of such a piezoelectric transformer has been disclosed in a Japanese monthly magazine, NIKKEI ELECTRONICS, Nov. 7, 1994 (No. 621), pages 147 to 157.
The configuration of the piezoelectric transformer-driving circuit in accordance with the above-mentioned prior art will be described briefly below by referring to FIG. 62. FIG. 62 is a block diagram showing the configuration of the above-mentioned conventional piezoelectric transformer-driving circuit.
In FIG. 62, a piezoelectric transformer 101 is a voltage transformation device for obtaining an amplified voltage. A transformer with winding 102 disposed ahead of the piezoelectric transformer 101 and used as a pre-transformer is an electromagnetic transformer provided to supplement the insufficient voltage step-up of the piezoelectric transformer 101. The voltage from the piezoelectric transformer 101 is applied to a cold cathode tube 103. A current detector 104 detects the current flowing through the cold cathode tube 103 and converts it into a voltage signal. A rectifying circuit 105 rectifies the sine wave like AC voltage output from the current detector 104, and converts it into a detection signal of a near DC voltage. A first adder 107 calculates the balance voltage between the detection signal output from the rectifying circuit 105 and a brightness setting voltage, that is, reference data (a reference voltage) which is input externally. An integrator 150 used as a filter circuit integrates the balance voltage output from the first adder 107 and converts it into a DC voltage.
A second adder 180 adds the DC voltage, which is the output of the integrator 150, to an initial value for determining the oscillation frequency of the piezoelectric transformer 101 at the time of power-on, and outputs a frequency setting voltage. A V-F converter 190 oscillates at a frequency corresponding to the above-mentioned frequency setting voltage. The V-F converter 190 has been set so that the oscillation frequency becomes high when the frequency setting voltage is negative, and so that the oscillation frequency becomes low when the frequency setting voltage is positive. Furthermore, the V-F converter 190 has been set to oscillate at a frequency sufficiently higher than the resonance frequency of the piezoelectric transformer 101 at the time of power-on. A driving circuit 110 formed of a power transistor amplifies the signal output from the V-F converter 190 to drive the winding transformer 102.
In the driving circuit of the piezoelectric transformer 101 configured as described above, at the time of power-on, the V-F converter 190 oscillates at a frequency higher than the resonance frequency of the piezoelectric transformer 101, and a voltage lower than the voltage level of the resonance frequency is output from the secondary electrode of the piezoelectric transformer 101. The voltage output from the secondary electrode of the piezoelectric transformer 101 is applied to the cold cathode tube 103.
A current in proportion to the applied voltage flows through the cold cathode tube 103, and the current flowing through the cold cathode tube 103 is converted into a voltage by the current detector 104, and is further converted into a nearly DC voltage by the rectifying circuit 105.
In the above-mentioned configuration, when the reference voltage to be supplied externally has a voltage level shown at point A of the characteristic curve shown in FIG. 61 and when the voltage obtained from the rectifying circuit 105 is the voltage at point B of the characteristic curve shown in FIG. 61, the balance voltage at the first adder 107 becomes positive. For this reason, the input voltage of the V-F converter 190 rises gradually, and the oscillation frequency output from the V-F converter 190 begins to lower. This operation raises the level of the voltage output from the piezoelectric transformer 101 and increases the current flowing through the cold cathode tube 103.
On the other hand, when the voltage obtained from the rectifying circuit 105 is the voltage at point C of the characteristic curve shown in FIG. 61, the balance voltage at the first adder 107 becomes negative, and the oscillation frequency output from the V-F converter 190 rises. This operation lowers the voltage level output from the piezoelectric transformer 101 and reduces the current flowing through the cold cathode tube 103.
As described above, the conventional piezoelectric transformer-driving circuit feeds back the current flowing through the cold cathode tube 103 and controls the oscillation frequency so that the current value is equal to the level of the reference data supplied externally thereby stabilizing the brightness of the backlight.
Furthermore, as another method of stabilizing the brightness of the backlight, methods have been disclosed in the Japanese published unexamined Utility Model Application No. Hei 4-58085 and the Japanese published unexamined Utility Model Application No. Hei 5-4779. In both of these methods, control is carried out at the resonance frequency at which the piezoelectric device has the highest efficiency. The principle thereof uses the fact that the phase difference between the voltage phase and the current phase obtained from the secondary side of the piezoelectric device becomes zero at the resonance frequency; the phase difference between the two is detected and frequency control is carried out so that the difference becomes zero. In this case, the output voltage is fixed at the output voltage obtained at the resonance point, therefore, it is general that the method of adjusting the brightness of the backlight is carried out by adjusting the voltage of the power supply.
When installing this kind of conventional piezoelectric transformer-driving circuit in compact video apparatuses such as a VCR integrated with a camera, a digital camera and the like, the apparatuses are desired to be made more compact so that the portability of the compact video apparatuses can be enhanced. For this reason, it is necessary to reduce the number of parts and to save space. However, since the conventional piezoelectric transformer-driving circuit is formed of an analog circuit, it has been difficult to reduce the number of parts.
Therefore, the piezoelectric transformer-driving circuit is desired to be digitalized in order to reduce the number of parts. However, the following requirements must be satisfied to digitalize the conventional piezoelectric transformer-driving circuit formed of an analog system.
(1) A high-frequency clock is required to obtain the frequency accuracy required for controlling the piezoelectric transformer.
(2) A multi-bit A/D converter is required to obtain voltage detection performance nearly equivalent to that of the analog system, therefore, an expensive LSI is required.
The above-mentioned requirements (1) and (2) will be described specifically.
Regarding the requirement (1), as a method of generating the drive pulse signal for the piezoelectric transformer, a method of obtaining a drive pulse signal having a desired frequency by frequency-dividing the clock is available. In this method of frequency-dividing the clock, the frequency of the clock required to control the current of the cold cathode tube in a predetermined range (xc2x11% for example) is simply obtained from the frequency characteristic curve showing the relationship between the output (dB) and the drive frequency (Hz) at the secondary electrode of the piezoelectric transformer in FIG. 61.
In a typical piezoelectric transformer, its resonance frequency is about 100 KHz, and the average frequency sensitivity in the frequency range (on the higher frequency side from the resonance frequency) to be controlled is to the extent that the change in the voltage value is about nearly +100% to xe2x88x9250% depending on the frequency change of 1 KHZ. Since the current flowing through the cold cathode tube is proportional to the voltage of the secondary electrode of the piezoelectric transformer, the voltage of the secondary electrode of the piezoelectric transformer is required to be limited within about xc2x11% in order to limit the value of the current flowing through the cold cathode tube within xc2x11%.
In order to reduce the voltage of the secondary electrode of the piezoelectric transformer within about xc2x11%, the frequency resolution of the drive pulse signal is required to be at least about 10 to 20 Hz. The clock frequency required to obtain a frequency resolution of 10 to 20 Hz at about 100 KHZ is 500 MHz to 1 GHz. This kind of high clock frequency is not a practical clock frequency in consideration of emission interference, supply power and the like.
Regarding the above-mentioned requirement (2), a current detector having a detection accuracy of xc2x11% or less is required to control the current of the cold cathode tube within a predetermined range (xc2x11% for example just as in the above-mentioned case). Furthermore, more accurate detection is necessary in consideration of the variations in the current detector and the rectifying circuit. Therefore, in order to control the current of the cold cathode tube within the predetermined range, an 8 or 9-bit A/D converter is required eventually. If such a converter is built in an LSI, the area of the chip becomes large, thereby being disadvantageous in cost.
The present invention has been devised to solve the above-mentioned problems, and is intended to propose a piezoelectric transformer-driving circuit capable of forming a drive pulse signal having a high frequency resolution from a clock having a low frequency and capable of detecting a voltage suited for digital processing by using a simple low-cost configuration.
In order to attain the above-mentioned objects, a piezoelectric transformer-driving circuit in accordance with the present invention comprises:
a piezoelectric transformer supplied with a controlled current to be supplied to a cold cathode tube and driven at the frequency of a drive pulse signal,
a current detector for detecting a load current flowing through the above-mentioned cold cathode tube,
a rectifying circuit for converting a sine wave like AC voltage obtained from the above-mentioned current detector into a substantially DC voltage,
an A/D converter for converting the voltage signal rectified by the above-mentioned rectifying circuit into a digital signal,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned A/D converter and reference data supplied externally by a constant and for outputting the result as error data,
a frequency setting circuit for setting the frequency of the drive pulse signal of the piezoelectric transformer as M-bit data (M: an integer) depending on the above-mentioned error data,
a divider circuit for dividing a clock having a predetermined frequency at a predetermined frequency division ratio to generate the drive pulse signal of the piezoelectric transformer, for distributing the above-mentioned frequency division ratio for the period of N cycles (N: an integer) of the drive pulse signal of the above-mentioned piezoelectric transformer and for carrying out control so that the average frequency division ratio in the period of N cycles is substantially equal to the value obtained by dividing the M-bit data output from the above-mentioned frequency setting circuit by N, and
a power transistor for driving the above-mentioned piezoelectric transformer.
In accordance with the present invention configured as described above, the frequency division ratio of the drive pulse signal is distributed, and therefore, the resolution of the average frequency is improved, and it is possible to obtain a piezoelectric transformer-driving circuit capable of obtaining a frequency for setting the brightness of the cold cathode tube at a desired value.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view comprises:
a piezoelectric transformer supplied with a controlled current to be supplied to a cold cathode tube and driven at the frequency of a drive pulse signal,
a current detector for detecting a load current flowing through the above-mentioned cold cathode tube,
a rectifying circuit for converting a sine wave like AC voltage obtained from the above-mentioned current detector into a substantially DC voltage,
an A/D converter for converting the voltage signal rectified by the above-mentioned rectifying circuit into a digital signal,
an error voltage calculation circuit for multiplying by a constant the difference data between the output data of the above-mentioned A/D converter and the reference data supplied externally, thereby outputting the result as error data,
a frequency setting circuit for setting the frequency of the drive pulse signal of the piezoelectric transformer as M-bit data (M: an integer) in accordance with the above-mentioned error data,
a divider circuit for dividing a clock having a predetermined frequency to generate the drive pulse signal of the piezoelectric transformer, and
a power transistor for driving the above-mentioned piezoelectric transformer, wherein
the frequency division ratio of the above-mentioned divider circuit is distributed in the period of predetermined cycles, and the frequency division ratio at the (A0xc2x720+A1xc2x721+ . . . +Anxe2x88x921xc2x72nxe2x88x921)th cycle (wherein, in the above-mentioned expression, Ax represented by A0, A1, . . . Anxe2x88x921 is a numerical value, 0 or 1) is given from the low-order n-bit data (B0xc2x720+B1xc2x721+ . . . +Bnxe2x88x921xc2x72nxe2x88x921; where, in the above-mentioned expression, Bx represented by B0, B1, . . . , Bnxe2x88x921 is a numerical value, 0 or 1) and the high-order m-bit data C (C: a decimal number) of the M-bit data output from the above-mentioned frequency setting circuit by the following expression (5):
{A0xc2x7Bnxe2x88x921+{overscore (A+L 0)}xc2x7A1xc2x7Bnxe2x88x922+ . . . +{overscore (A+L 0)}xc2x7A1xc2x7 . . . Anxe2x88x922xc2x7Anxe2x88x921xc2x7B0}+Cxe2x80x83xe2x80x83(5).
In the expression (5), xe2x80x9c{overscore (Ax)}xe2x80x9d represents the inversion of xe2x80x9cAxxe2x80x9d (1 is converted into 0, and 0 is converted into 1), and the upper bars in the following descriptions are used to represent this meaning.
In accordance with the present invention configured as described above, the levels of the low frequency components become lower, and therefore, it is possible to obtain a piezoelectric transformer-driving circuit capable of suppressing the flicker phenomenon that is apt to appear as the frequency is lower or as the level of the change in brightness is larger.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view comprises:
a piezoelectric transformer supplied with a controlled current to be supplied to a cold cathode tube and driven at the frequency of a drive pulse signal,
a current detector for detecting a load current flowing through the above-mentioned cold cathode tube,
a rectifying circuit for converting a sine wave like AC voltage obtained from the above-mentioned current detector into a substantially DC voltage,
an A/D converter for converting the rectified voltage signal into a digital signal by using a predetermined sampling clock,
a smoothing circuit for smoothing the output data of the above-mentioned A/D converter in a predetermined cycle,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned smoothing circuit and reference data supplied externally by a constant and for outputting the result as error data,
a frequency setting circuit for setting the frequency of the drive pulse signal of the piezoelectric transformer as M-bit data depending on the above-mentioned error data,
a divider circuit for dividing a clock having a predetermined frequency to generate the drive pulse signal of the piezoelectric transformer, and
a power transistor for driving the above-mentioned piezoelectric transformer.
In accordance with the present invention configured as described above, a high voltage detection resolution can be obtained even if a low-bit A/D converter is used, thereby having a great effect in reducing cost, in addition, it is not necessary to install a large-capacitance capacitor, and therefore, the space for parts can be saved, thereby being highly effective in making compact video apparatuses more compact.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view comprises:
a piezoelectric transformer supplied with a controlled current to be supplied to a cold cathode tube and driven at the frequency of a drive pulse signal,
a current detector for detecting a load current flowing through the above-mentioned cold cathode tube,
a half-wave rectifying circuit for half-wave rectifying a sine wave like AC voltage obtained from the above-mentioned current detector,
a comparator for comparing the half-wave rectified voltage signal with a predetermined reference voltage and for outputting xe2x80x9cHxe2x80x9d level data or xe2x80x9cLxe2x80x9d level data,
a pulse width detection circuit for detecting the pulse width of the output data of the above-mentioned comparator,
a smoothing circuit for smoothing the pulse width data output from the above-mentioned pulse width detection circuit in a predetermined cycle,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned smoothing circuit and reference data supplied externally by a constant and for outputting the result as error data,
a frequency setting circuit for multiplying the output data of the above-mentioned smoothing circuit by a constant and for setting the frequency of the drive pulse signal of the piezoelectric transformer as M-bit data (M: an integer),
a divider circuit for dividing a clock having a predetermined frequency to generate the drive pulse signal of the piezoelectric transformer, and
a power transistor for driving the above-mentioned piezoelectric transformer.
In accordance with the present invention configured as described above, voltage detection is carried out by using the half-wave rectifying circuit and the comparator, therefore, voltage detection is made possible with high accuracy by using a simple and low-cost configuration, whereby the effect thereof is significant in the digitalization of the piezoelectric transformer-driving circuit.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view comprises:
a piezoelectric transformer supplied with a controlled current to be supplied to a cold cathode tube and driven at the frequency of a drive pulse signal,
a current detector for detecting a load current flowing through the above-mentioned cold cathode tube,
a rectifying circuit for converting a sine wave like AC voltage obtained from the above-mentioned current detector into a substantially DC voltage,
a comparator for comparing the rectified voltage signal with a predetermined reference voltage and for outputting xe2x80x9cHxe2x80x9d level data or xe2x80x9cLxe2x80x9d level data,
a counter circuit for counting the output data of the above-mentioned comparator for a predetermined period,
a smoothing circuit for smoothing the count data output from the above-mentioned counter circuit in a predetermined cycle,
a selector circuit for selecting the output data of the above-mentioned smoothing circuit or the output data of the above-mentioned counter circuit and for outputting the selected data,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned selector circuit and reference data supplied externally by a constant and for outputting the result as error data,
a frequency setting circuit for setting the frequency of the drive pulse signal of the piezoelectric transformer as M-bit data (M: an integer) depending on the above-mentioned error data,
a divider circuit for dividing a clock having a predetermined frequency to generate the drive pulse signal of the piezoelectric transformer, and
a power transistor for driving the above-mentioned piezoelectric transformer.
In accordance with the present invention configured as described above, voltage detection values can be digitalized with high accuracy by a simple configuration using the comparator, in addition, a significant effect is obtained for the digitalization of the piezoelectric transformer-driving circuit.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view, wherein the output voltage thereof is changed by changing the frequency of a drive pulse signal to control the current flowing through a cold cathode tube, comprises:
a current detection circuit for detecting a load current, a rectifying circuit for converting a sine wave like voltage obtained from the above-mentioned current detection circuit into a DC voltage,
an A/D converter for converting the rectified voltage signal into a digital signal,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned A/D converter and reference data supplied externally by a constant and for outputting the result as error data,
a frequency setting circuit for setting the frequency of the drive pulse signal of the piezoelectric transformer as M-bit data depending on the above-mentioned error data,
a divider circuit for performing frequency division at the positive edges of a clock having a predetermined frequency to generate the drive pulse signal of the piezoelectric transformer,
a reverse-edge processing circuit for latching the output pulse signal of the above-mentioned divider circuit at the reverse edges of the above-mentioned clock and for outputting the latched signal,
a selector for selecting the drive pulse signal output from the above-mentioned divider circuit or the drive pulse signal output from the above-mentioned reverse-edge processing circuit and for outputting the selected pulse signal, and
a power transistor for driving the piezoelectric transformer, wherein
the frequency division ratio of the above-mentioned divider circuit is distributed for the period of N cycles of the drive pulse signal of the above-mentioned piezoelectric transformer so that the average frequency division ratio for the period of N cycles is substantially equal to the value obtained by dividing the M-bit data output from the above-mentioned frequency setting circuit by N and so that the fluctuation of the frequency division ratio for the period of N cycles is one or less.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view, wherein the output voltage thereof is changed by changing the frequency of a drive pulse signal to control the current flowing through a cold cathode tube, comprises:
a current detection circuit for detecting a load current,
a rectifying circuit for converting a sine wave like voltage obtained from the above-mentioned current detection circuit into a DC voltage,
an A/D converter for converting the rectified voltage signal into a digital signal,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned A/D converter and reference data supplied externally by a constant and for outputting the result as error data,
a frequency setting circuit for setting the frequency of the drive pulse signal of the piezoelectric transformer as M-bit data depending on the above-mentioned error data,
a divider circuit for dividing a clock having a predetermined frequency to generate the drive pulse signal of the piezoelectric transformer,
a pulse width setting circuit for setting the duty ratio of the drive pulse signal output from the above-mentioned divider circuit depending on power supply voltage information from an external microcomputer system, and
a power transistor for driving the piezoelectric transformer.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view, wherein the output voltage thereof is changed by changing the frequency of a drive pulse signal to control the current flowing through a cold cathode tube, comprises:
a current detection circuit for detecting a load current,
a rectifying circuit for converting a sine wave like voltage obtained from the above-mentioned current detection circuit into a DC voltage,
an A/D converter for converting the rectified voltage signal into a digital signal,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned A/D converter and reference data supplied externally by a constant and for outputting the result as error data,
a peak detection circuit for holding the output data of the above-mentioned A/D converter for a predetermined period and for comparing the output data with previous data to detect the resonance point of the piezoelectric transformer,
a polarity inversion circuit for inverting the polarity of the above-mentioned error data depending on the output data of the above-mentioned peak detection circuit
a frequency setting circuit for setting the frequency of the drive pulse signal of the piezoelectric transformer by adding the output data of the above-mentioned polarity inversion circuit to the previous frequency setting value,
a divider circuit for dividing a clock having a predetermined frequency at a frequency division ratio depending on the output data of the above-mentioned frequency setting circuit to generate the drive pulse signal of the piezoelectric transformer, and
a power transistor for driving the above-mentioned piezoelectric transformer.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view, wherein the output voltage thereof is changed by changing the frequency of a drive pulse signal to control the current flowing through a cold cathode tube, comprises:
a current detection circuit for detecting a load current,
a rectifying circuit for converting a sine wave like voltage obtained from the above-mentioned current detection circuit into a DC voltage,
an A/D converter for converting the rectified voltage signal into a digital signal,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned A/D converter and reference data supplied externally by a constant and for outputting the result as error data,
a peak detection circuit for holding the output data of the above-mentioned A/D converter for a predetermined period and for comparing the output data with previous data to detect the resonance point of the piezoelectric transformer,
a frequency setting circuit for adding the above-mentioned error data to the previous frequency setting data and for outputting the result,
a selector for selecting the output of the above-mentioned frequency setting circuit or the frequency setting data at the previous N-th time and for outputting the selected data as the frequency setting data of the drive pulse signal of the piezoelectric transformer,
a divider circuit for dividing a clock having a predetermined frequency at a frequency division ratio depending on the output data of the above-mentioned selector to generate the drive pulse signal of the piezoelectric transformer, and
a power transistor for driving the above-mentioned piezoelectric transformer.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view, wherein the output voltage thereof is changed by changing the frequency of a drive pulse signal to control the current flowing through a cold cathode tube, comprises:
a current detection circuit for detecting a load current,
a voltage detection circuit for detecting the output voltage of the piezoelectric transformer,
a rectifying circuit for detecting the sine wave like voltage obtained from the above-mentioned current detection circuit or the sine wave like voltage obtained from the above-mentioned voltage detection circuit, whichever larger, and for converting the voltage into a DC voltage,
a turning-off circuit for periodically turning off the output of the above-mentioned voltage detection circuit for a predetermined period,
an A/D converter for converting the rectified voltage signal into a digital signal,
a lighting detection circuit for detecting the output voltage of the A/D converter at the time when the above-mentioned turning-off circuit is on and for judging the lighting of the cold cathode tube,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned A/D converter and reference data supplied externally by a constant and for outputting the result as error data,
a selector for selecting the above-mentioned reference data at the time of lighting or the above-mentioned reference data at the time of start and for outputting the selected data,
a frequency setting circuit for setting an initial frequency at the time of start and for adding the above-mentioned error data to the previous frequency setting data and outputting the result at the next time and thereafter,
a divider circuit for dividing a clock having a predetermined frequency at a frequency division ratio depending on the output data of the above-mentioned frequency setting circuit to generate the drive pulse signal of the piezoelectric transformer, and
a power transistor for driving the above-mentioned piezoelectric transformer, wherein
the frequency of the drive pulse signal of the above-mentioned piezoelectric transformer is held constant at the time when the above-mentioned turning-off circuit is on.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view, wherein the output voltage thereof is changed by changing the frequency of a drive pulse signal to control the current flowing through a cold cathode tube, comprises:
a current detection circuit for detecting a load current,
a rectifying circuit for detecting the sine wave like voltage obtained from the above-mentioned current detection circuit and for converting the voltage into a DC voltage,
an A/D converter for converting the rectified voltage signal into a digital signal,
a lighting detection circuit for detecting the output voltage of the A/D converter and for judging the lighting of the cold cathode tube,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned A/D converter and reference data supplied externally by a constant and for outputting the result as error data,
a frequency setting circuit for setting an initial frequency at the time of start and for adding the above-mentioned error data to the previous frequency setting data and outputting the result at the next time and thereafter,
a divider circuit for dividing a clock having a predetermined frequency at a frequency division ratio depending on the output data of the above-mentioned frequency setting circuit to generate the drive pulse signal of the piezoelectric transformer,
an output enable circuit for controlling the output of the drive pulse signal output from the above-mentioned divider circuit,
a power transistor for driving the above-mentioned piezoelectric transformer, and
a restart processing circuit that disables the above-mentioned output enable circuit if the above-mentioned lighting detection circuit does not judge that lighting is attained even after a predetermined period has passed and sets the frequency setting data output from the above-mentioned frequency setting circuit at the initial frequency to perform restart processing.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view, wherein the output voltage thereof is changed by changing the frequency of a drive pulse signal to control the current flowing through a cold cathode tube, comprises:
a current detection circuit for detecting a load current,
a voltage detection circuit for detecting the output voltage of the piezoelectric transformer,
a rectifying circuit for detecting the sine wave like voltage obtained from the above-mentioned current detection circuit or the sine wave like voltage obtained from the above-mentioned voltage detection circuit, whichever larger, and for converting the voltage into a DC voltage,
a turning-off circuit for periodically turning off the output of the above-mentioned voltage detection circuit for a predetermined period,
an A/D converter for converting the rectified voltage signal into a digital signal,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned A/D converter and reference data supplied externally by a constant and for outputting the result as error-data,
a frequency setting circuit for setting an initial frequency at the time of start and for adding the above-mentioned error data to the previous frequency setting data and outputting the result at the next time and thereafter,
a divider circuit for dividing a clock having a predetermined frequency at a frequency division ratio depending on the output data of the above-mentioned frequency setting circuit to generate the drive pulse signal of the piezoelectric transformer,
an output enable circuit for controlling the output of the drive pulse signal output from the above-mentioned divider circuit,
an abnormality detection circuit for detecting the output voltage A of the above-mentioned A/D converter when the above-mentioned turning-off circuit is on or for detecting the output voltage B of the above-mentioned A/D converter when the above-mentioned turning-off circuit is off,
a protection circuit having functions for outputting a reset signal used to switch the frequency data of the above-mentioned frequency setting circuit to the initial frequency for a predetermined period and to disable the above-mentioned output enable circuit when the above-mentioned output voltage B is less than a predetermined level, and for disabling the output enable circuit to stop outputting when the output voltage A is less than the predetermined level and the output voltage B is not less than the predetermined level, and
a power transistor for driving the above-mentioned piezoelectric transformer.
A piezoelectric transformer-driving circuit in accordance with the present invention from another point of view, wherein the output voltage thereof is changed by changing the frequency of a drive pulse signal to control the current flowing through a cold cathode tube, comprises:
a current detection circuit for detecting a load current,
a rectifying circuit for converting the sine wave like voltage obtained from the above-mentioned current detection circuit into a DC voltage,
an A/D converter for converting the rectified voltage signal into a digital signal,
an error voltage calculation circuit for multiplying the difference data between the output data of the above-mentioned A/D converter and reference data supplied externally by a constant and for outputting the result as error data,
a clip circuit for clipping the above-mentioned error data value in a predetermined range,
a frequency setting circuit for adding the output data of the above-mentioned clip circuit to the previous frequency setting data and for outputting the result as the frequency setting data of the drive pulse signal of the piezoelectric transformer,
a divider circuit for dividing a clock having a predetermined frequency at a frequency division ratio depending on the above-mentioned frequency setting data to generate the drive pulse signal of the piezoelectric transformer, and a power transistor for driving the above-mentioned piezoelectric transformer.
In accordance with the present invention configured as described above, an analog detection voltage can be converted into a highly accurate digital signal by using a simple low-cost configuration while highly accurate control performance is attained, thereby being effective in obtaining a piezoelectric transformer-driving circuit for greatly contributing to enhancing cost merit in the LSI formation of the driving circuit, thereby being greatly effective in making compact video apparatuses more compact and lower in cost.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.